1. Field of the Invention
The invention relates to a process or the ring-monochlorination of 4-nitro-alkylbenzenes in the presence of Friedel-Crafts catalysts and simultaneous presence of sulphur-containing, benzo-condensed heterocyclic co-catalysts in the liquid phase.
2. Description of the Related Art
The reaction of 4-nitrotoluene with gaseous chlorine in the presence of Friedel-Crafts catalysts such as iron-III chloride (U.S. Pat. No. 3 341 595; J. Chem Soc. 1927, 2905) or antimony chlorides (Bull. Soc. Chim. Belg. 61 (1952), 317) is known. In addition to the desired 2-chloro-4-nitrotoluene, more highly chlorinated, positionally isomeric dichloro-4-nitrotoluenes and 4-nitro-benzyl chloride, as well as the ring-chlorinated derivatives derived from this, are observed as by-products.
Thus, a selectivity of about 92% for 2-chloro-4-nitrotoluene is observed when iron-III chloride is used as catalyst (see Example 2), which gave rise to the desire for further lowering, in particular of 2,6-dichloro-4-nitrotoluene and 4-nitro-benzyl chloride. It has been attempted further to increase the selectivity in the desired direction with the aid of co-catalysts. In this connection iron or iron-III chloride have particularly frequently been used with iodine as co-catalysts. In (Naturwiss. 17 (1929), 13; Houben-Weyl, Methoden der Organischen Chemie (Methods in Organic Chemistry), volume V/3 (1962), 704; JP-B-75/7589; CS 193 662). An iron/iodine/PCl.sub.3 catalyst combination in the ratio by weight of 60:1:2 is disclosed in U.S. Pat. No. 3 005 031.
The selectivity for 2-chloro-4-nitrotoluene can indeed be increased to above 96% by the co-use of iodine (see Example 3), but this is at the cost of considerable difficulties. On the one hand the iodine introduced is discharged with the abundantly formed hydrogen chloride in the form of volatile compounds, for example as hydrogen iodide, which impairs the working up and the purity of the hydrochloric acid produced as by-product. Additionally, traces of iodine in the bound form, which cannot be removed by physical separation methods, remain in the chlorination mixture and in the end product 2-chloro-4-nitrotoluene. These impurities interfere considerably in the further processing of the distilled 2-chloro-4-nitrotoluene also and therefore signify a considerable reduction in the product quality. When working up such chlorination mixtures by distillation elemental iodine can sublime in more distant parts of the distillation apparatus; additionally formation of iodine-containing working-up residues occurs, which is not very favourable.
A process disclosed in DE-OS 3 128 442 for the chlorination of 4-nitrotoluene using iodine as the sole catalyst in an amount of, for example, 1% by weight, relative to the starting material, shows the described disadvantages of the use of iodine to a particular degree because of the increased quantity. Additionally, in this last-mentioned process up to 75% of the chlorine introduced escapes unused from the reaction mixture; consequently an excess of up to 300% above the required amount of chlorine will be necessary before an industrially required conversion of, for example, more than 90% is achieved.
A further process for the chlorination of aromatic hydrocarbons, amongst which 4-nitrotoluene is also listed, is described in CA 1 176 654; this process is carried out in a mixture with 96.5% sulphuric acid (ratio by weight H.sub.2 SO.sub.4 :4-nitrotoluene=1:1.35) with or without addition of small amounts of Al.sub.2 O.sub.3. Further composition of the chlorination mixture or of the yield are not given. However, processes using such amounts of sulphuric acid are extremely unfavourable technologically, because of the corrosion, and environmentally, because of the waste disposal. Additionally, in process engineering an aqueous working up to remove the sulphuric acid is required before any precision distillation.
There was therefore the desire to find catalyst systems to increase the selectivity for 2-chloro-4-nitroalkylbenzenes, i.e. to suppress the said undesired by-products, in particular the higher levels of chlorination, which do not possess the disadvantages described above. It was possible to achive this object in an extremely favourable manner by the combination of Friedel-Crafts catalysts with the dibenzo-condensed sulphur heterocycles described further below as co-catalysts.